Elastic Nonlinearity of Vitreous Silica at Low Temperatures

Abstract

Low-temperature anomalies in the thermal, acoustic and dielectric properties of amorphous solids have found increasing attention in recent years.¹ Compared with crystals, these materials exhibit an anomalously high specific heat and low thermal conductivity². This behaviour can be explained by the existence of low-energy excitations with a nearly constant density of states in an energy range extending at least between 10 mK and 10 K. More detailed information has been obtained by ultrasonic and dielectric measurements³ which demonstrate that these excitations are best described as two-level systems. According to a generally accepted model they originate from small groups of atoms, which are energetically nearly equivalent, tunnelling between two positions in the amorphous matrix.⁴ About the physical nature of the tunnelling “particles”, however, not much has previously been known.